1. Field of the Invention
The present invention relates to a paper detecting device and a printer incorporating the paper detecting device, particularly to an improvement in a movable paper detecting device.
2. Description of the Prior Art
A printer generally includes a paper detecting device to detect a paper and paper reference positions in order to properly set paper feed timing and print start/end timing and print information on the paper at the right position while feeding the paper.
This paper detecting device includes a paper detector such as a reflective or transmissive type optical sensor. When mounted in a printer in which papers in different widths are selectively used, the paper detector is set to be movable in paper width direction in line with the type of a paper in use, for example.
A printer dedicated for a single paper size may include the movable paper detector so as to accurately detect black marks indicating paper reference positions which are defined differently depending on a paper.
Such a paper detecting device includes a guide element having a path along which the paper detector is movable or an operator can move it to an appropriate position.
In prior art Japanese Patent Application Publication No. 2003-146482 (Reference 1), for example, discloses a paper detecting device configured to be supported by the guide element via an elastic element. This paper detecting device is fixed in a predetermined position on the guide element by the contact friction of the elastic element, while it can be moved along the path, applied with an operational force against the friction.
Also, Japanese Patent Application Publication No. 2001-310523 (Reference 2) discloses a paper detecting device which includes a paper detector having a protrusion and a guide element with an uneven portion in which concaves and convexes are alternately arranged along the path so that the protrusion is fitted into any of the concaves. Applied with an operational force, the protrusion is released from the engagement and the paper detector is moved along the path while the protrusion is fitted into the convexes in order. With this device, an operator can feel a sense of clicking every time the protrusion is fitted into the concave.
However, the device disclosed in Reference 1 faces a problem that it is difficult to precisely stop the paper detector at a desired position since applied with an initial force to move the paper detector in a still state, it may be moved too far unexpectedly. This occurs because the friction of the elastic element needs to be set relatively large in order to securely stop the paper detector while no force is applied, and dynamic friction as a resistance of the detector moving along the path is smaller than static friction as that of the detector immediately before moving.
Meanwhile, if the elastic element is set to have a small friction, an operator's careless touch may easily move the paper detector and the paper detector cannot be held at a predetermined stop position reliably.
In view of the above problems, the device can be configured to additionally include a friction release mechanism to allow an operator to stop the paper detector at a desired position by removing the frictional resistance with the release mechanism before applying a force to move it. However, this may cause another problem that with no indication or marks for stop positions provided, the operator cannot know the right position to stop the detector so that he or she operates it only by his/her experience or intuition and the desired position at which the operator has stopped the detector may not be actually the right stop position originally set in the device.
Further, in the device disclosed in Reference 2, the concave of the guide element and the protrusion of the paper detector are configured to be relatively strongly engaged with each other to securely hold the paper detector in the stop state.
However, with the concave and the protrusion strongly engaged, the paper detector cannot be smoothly moved when applied with an operational force. In addition, once it starts moving, it may be moved too far, passing over a target stop position. Meanwhile, when the engagement is weak, the paper detector may be unintentionally moved by unexpected contact with another element or an operator's finger or else.